1. Field of the Invention
This invention is directed to preparation of orthoacetic acid alkyl esters. More especially, this invention is directed to the preparation of such orthoacetic acid alkyl esters from acetonitrile, dry hydrogen chloride and an anhydrous alkanol followed by alcoholysis of the resultant intermediate imidoester hydrochloride. This invention is particularly concerned with the supression of secondary reactions which reduce the yields of the desired orthoacetic acid alkyl ester. This invention is therefore particularly concerned with the realization of sufficiently high yields or orthoacetic acid alkyl esters through such a route that the process can be economically performed on a commercial scale.
2. Discussion of the Prior Art
The preparation of orthoacetic acid alkyl esters of the formula ##STR2## in which R is a saturated, branched or unbranched, alkyl moiety of 1 to 8 carbon atoms is known. Furthermore, it is known to react acetonitrile with the corresponding anhydrous alcohol and dry hydrogen chloride in a two-step process in the presence of an organic solvent, the reaction to the corresponding imidoester hydrochloride taking place in the first step according to equation I below. In the second step the imidoester hydrochloride intermediate is subjected, while in the acid-free state, to an alcoholysis whereby the desired orthoacetic acid alkyl ester is formed in admixture with by-product ammonium chloride. The ammonium chloride is removed and following the ammonium chloride removal the reaction product is subjected to distillation in order to recover the desired orthoacetic acid alkyl ester. The equations for the process are as follows: ##STR3##
The preparation of orthocarboxylic acid esters by the reaction of carboxylic acid nitriles with alkanols and hydrogen chloride to form the corresponding imidoalkyl ester hydrochloride and the alcoholysis of the latter, is known (cf. H. Meerwein in Houben-Weyl, Vol. 6/3, pages 300-324, 1965). The yields in this very generally applicable method are impaired to one extent or another by a number of secondary reactions. The imidoalkyl ester hydrochloride is very sensitive to heat, so that it can be degraded to acetamide and alkyl chloride (Equation III). It reacts with alcohol in two secondary reactions to dialkyl ether, acetamide and hydrogen chloride (Equation IV), and also to dialkyl ether, acetic acid alkyl ester and ammonium chloride (Equation V). The formation of alkyl chloride is also possible through a nucleophilic substitution reaction between the alcohol and the hydrochloric acid (Equation VI). ##STR4##
In order to obtain a high orthoester yield, it is indispensable to suppress these secondary reactions.
In the literature citation given above, it is proposed on page 300 that the preparation of the heat-sensitive imidoester hydrochloride in accordance with Equation I be performed at very low temperatures, namely at 0.degree. to -30.degree. C. Ether is recommended as the organic solvent. The imidoester hydrochloride precipitated after the reaction mixture has been let stand for 48 hours is removed by suction filtering and washed with ether chilled to -40.degree. C., and digested repeatedly with chilled ether (-40.degree. C.) until free of acid, and then dried.
For the alcoholysis of the imidoester hydrochloride in accordance with Equation II, the procedure according to page 302 of the above-cited literature is to heat at ebullition (46.degree. C.) for 6 hours a mixture of 0.2 mole of imidoester hydrochloride with 3 moles of alcohol (7.5 times the stoichiometrically required amount of alcohol) with the addition of ether (volumetric ratio of alcohol to ether 1:1 ). The reaction mixture is then cooled to 0.degree. C. and filtered.
These procedures are not satisfactory as regards the volume-time yield. The yields are about 64 to 74%, with respect to the acetonitrile, and the reaction time--merely for the preparation of the imidoester hydrochloride--amounts to forty-eight hours.
It is furthermore disadvantage that the described methods are not suitable for the preparation of orthoacetic acid alkyl esters on a commercial scale, since they are very complex and expensive. Furthermore, working with such easily combustible substances as ether requires a considerable investment in apparatus for safety reasons.
It is an object of this invention, therefore, to provide a commercially feasible process wherein orthoacetic acid alkyl esters can be prepared in yields above 74 percent employing a substantially shorter reaction time. It is an object of this invention, therefore, to provide a process for the preparation of orthoacetic acid alkyl esters from acetonitrile, hydrogen chloride and an alkanol wherein the desired orthoacetic acid alkyl ester is recovered in a high volume-time yield. It is a further object of this invention to provide a process which does not require the use of ether or other dangerous solvents and which does not require extensive working-up techniques.